Inductive load drivers, more particularly ignition coil drivers, are generally designed to drive an inductive load over a very broad range of operating conditions. These operating conditions include extremes of ambient temperature, source voltage, engine RPM and load demand.
In an internal combustion engine based ignition system, a coil driver is employed to repetitively transfer energy from an electrical system to a primary winding of an ignition coil. At the appropriate time, this energy is transferred from the primary winding through a secondary winding to a spark plug where the energy is dissipated, igniting fuel in a cylinder.
During this process the amount of energy the coil driver transfers can be about 100 millijoules. In typical designs the coil driver, due to losses inherent in the design, will dissipate about 10 millijoules of this energy. Correspondingly, the average power dissipation of the coil driver is relatively small. This power dissipation is engine RPM dependent and typically is about 1.5 watts at 3,500 RPM. This results in an average rise in coil driver temperature of about 6 degrees Celsius.
If an ignition coil open secondary condition occurs, the energy stored in the primary winding of the ignition coil must be absorbed by the coil driver. This is because an impedance in the secondary winding, that under normal operating conditions is typically very low, is now very high. This results in the energy being reflected from the secondary winding back to the primary winding and finally back to the driver. An open secondary condition can occur many ways including; a disconnected spark plug wire, a cracked spark plug, or an opening of the secondary winding to cite a few. Because an open secondary condition can persist, the coil driver must be designed to have a reserve power dissipation capability to absorb the resulting excess energy without self destruction. This additional demand requires that the coil driver must withstand the 100 millijoules of energy normally transferred from the primary winding to the secondary winding, raising the expectation from 10 millijoules to 110 millijoules of energy. Repetitive application of this condition will cause the temperature of the coil driver to increase dramatically. Consequently, this additional demand corresponds to an approximate power dissipation of 8 watts at 3,500 RPM and an average rise in coil driver temperature of 32 degrees Celsius.
FIG. 1 illustrates a set of typical waveforms found in a coil driver. The set on the left represents coil driver waveforms found during normal operation of an ignition cycle. The set on the right represents coil driver waveforms found during an open secondary condition.
Referring to the normal operation of an ignition cycle, as shown in the leftmost waveforms, a CDO, or coil driver on, waveform 101 transits at 118 to an on-state 119. This causes a coil driver to provide current to a primary winding of an ignition coil. This primary winding current, I.sub.primary 105, builds from zero amps to about 5.5 amps as shown by reference number 120. This action builds the about 110 millijoules of energy in the primary winding mentioned earlier. Note that the CDO and I.sub.primary waveforms 101 and 105 are shown with an interrupted time line. This is done to maintain a time scale favoring illustration of the instantaneous behavior of various waveforms after a falling transition 121 of the CDO waveform 101. The waveforms shown after the transition at 121 are significantly shorter in duration than the waveforms shown before the transition. When the primary winding of the coil is fully charged, the CDO waveform 101 transits at 121 to an off-state 122. This causes several things to happen. A voltage, V.sub.primary 103, builds across the primary winding of the coil and is clamped at a voltage peak 123 of about 350 volts. This clamping is designed to protect the coil driver. V.sub.primary 103 is transformed by a secondary winding of the coil to a very high voltage, on the order of 30,000 volts, and fires a spark plug. During this time the energy in the secondary winding is primarily dissipated across the spark plug. Also, I.sub.primary 105 falls rapidly to zero, as shown by reference number 127, and V.sub.primary 103 falls rapidly to zero, oscillating slightly, as shown by a portion 125 of the waveform 103. Because the coil driver cannot force this current I.sub.primary 105 instantaneously to zero a small amount, typically 10 millijoules of energy is dissipated in the coil driver as described earlier. An E.sub.driver graph 115 shows this energy at reference number 131.
At the same time, an instantaneous power of about 1,600 watts is dissipated in the coil driver. A P.sub.driver graph 113 shows this power at reference number 129. Note that this corresponds to an average power dissipation of 1.5 watts at 3,500 engine RPM as mentioned earlier. This increase in power dissipation causes a rise in the temperature of the coil driver. A T.sub.driver graph 117 shows this rise in temperature at reference number 133. The average rise in temperature of the coil driver will be about 6 degrees Celsius as mentioned earlier.
Conversely, if the coil driver is driving a coil primary winding that has an open secondary winding, the second set of waveforms, on the right hand side of FIG. 1 apply. First, the CDO waveform 101 transits at 134 to an on-state 135 and I.sub.primary 105, builds from zero amps to about 5.5 amps as shown by a portion of 105 indicated by reference number 137. This action builds the about 110 millijoules of energy mentioned earlier. When the primary winding of the coil is fully charged, the CDO waveform 101 transits at 145 to an off-state 146. V.sub.primary 103 builds across the primary winding of the coil and is again clamped at a voltage 147 of about 350 volts. Because in this case the secondary winding of the coil is open the spark plug will not dissipate the energy and the about 110 millijoules of energy now in the primary winding of the coil must be dissipated in the coil driver. Consequently, the primary winding voltage V.sub.primary 103 does not fall rapidly to zero as it did earlier. Also, I.sub.primary 105 pertubates, as shown by waveform portion 151, and finally falls to zero, as shown by reference number 153. This pertubation is responsive to the impedance in the secondary winding 203. Notice that both the V.sub.primary waveform 103 and the I.sub.primary waveform 105 between the transition 145 to the off-state 146 of the CDO waveform 101 and reference number 153, are of a significantly longer duration than the waveforms found during normal operation of an ignition cycle detailed earlier.
Consequently, an instantaneous, and pertubating power of about 2,500 watts is dissipated over an elongated interval. This is shown in the P.sub.driver graph 113 at reference number 159. The energy dissipated in the coil driver is shown in the E.sub.driver graph 115 at reference number 163. Eventually, when the energy is dissipated in the coil driver, the instantaneous power dissipation falls to zero as shown by reference number 161. Note that this corresponds to an average power dissipation of 8 watts at 3,500 engine RPM as mentioned earlier. This increase in power dissipation causes a rise in the temperature of the coil driver. This is shown in the T.sub.driver graph 117 at reference number 165. This rise in temperature can peak at about 65 degrees Celsius before falling to zero, as shown by reference number 167. The average rise in temperature of the coil driver will be about 32 degrees Celsius as mentioned earlier.
In an automotive environment the coil driver is expected to operate at the high temperatures the engine must operate under. When the coil driver must dissipate additional energy, due to an open secondary condition, this will cause excessive high temperature operation of the coil driver. The heat that the coil driver must dissipate significantly reduces the reliable life of the coil driver. Additionally, large and bulky heat sinks are required to protect the coil driver from thermal destruction, making manufacturing and installation more difficult.
What is needed is an inductive load, or coil, driver that is adaptive to known variations in load conditions such that the load coil driver provides the drive required while avoiding high power dissipation in the driver and maximizing reliability, compactness, and ease of manufacture.